1. Field of the Invention
The present invention relates to noise generating apparatus, and more particularly to digital noise generating apparatus for producing rapidly changing discrete output frequencies.
2. Description of the Prior Art
Noise generating apparatus of the type that produces rapidly changing discrete output frequencies is well known in the art. Such apparatus has been used, for example, in suppressing the effectiveness of hostile radar. With the advent of more sophisticated radars operating at different unknown frequencies over various bandwidths, it is necessary to be able to generate different frequencies in random succession in different bandwidths rapidly in order to provide effective jamming of one or more radar installations. More recently, digital noise generators were developed which generate a psuedo-random sequence of digital binary numbers. This sequence is converted to a discretely varying analog voltage that is applied to a linearized voltage controlled oscillator (VCO). The output of the VCO is a flat spectrum with a bandwidth proportional to the peak to peak voltage swing of the analog voltage.
The flatness and the quality of the frequency spectrum at the output of the VCO is proportional to the ratio of the time at each frequency step to the VCO rise time or in other words the time to reach the next frequency step. Thus, when it is desired to generate random frequency steps at a rate of approximately six megahertz or 160 nanoseconds, with a relatively slow rise time of from thirty to fifty nanoseconds, for example, power is lost to unwanted frequencies; and the frequency spectrums tend to become distorted. Such distortion can be minimized, of course, by utilizing VCO's with extremely fast response times; i.e., less than ten nanoseconds, accurate linearity and precise calibration which substantially increases the cost of the system.
Therefore, it is desirable to provide a noise generating apparatus, which is capable of providing random frequency steps at a rapid rate with good spectral quality, a minimum of power loss due to unwanted frequencies; and yet be capable of utilizing less expensive voltage controlled oscillators having slower response times.